1. Field of the Invention
The present invention relates to a motion control apparatus for a vehicle which controls the motion of a vehicle by controlling the braking forces applied to the wheels of the vehicle.
2. Description of the Related Art
For example, a motion control apparatus (braking apparatus) for a vehicle disclosed in Japanese Patent Application Laid-Open (kokai) No. H10-81215 applies a braking force (hereinafter referred to “outer wheel braking force”) to a wheel (e.g., front wheel) located on the radially outer side of a turning locus when (the absolute value of) an actual lateral acceleration acting on the vehicle becomes equal to or higher than a certain threshold. With this configuration, upon application of the outer wheel braking force, a yawing moment in the direction opposite the turning direction of the vehicle is generated in the vehicle, whereby the magnitude of the actual lateral acceleration acting on the vehicle decreases, and thus, excessive rolling of the vehicle can be prevented.
However, when a yawing moment in the direction opposite the turning direction of the vehicle is generated in the vehicle upon application of the outer wheel braking force, the turning vehicle is controlled to a direction of understeering. As a result, in some cases a desired turning locus tracing performance cannot be maintained satisfactorily.
In order to overcome the above-described drawback, a motion control apparatus for a vehicle disclosed in Japanese Patent Application Laid-Open (kokai) No. 2005-35451 applies a braking force (hereinafter referred to “inner rear wheel braking force”) only to a rear wheel located on the radially inner side of a turning locus when (the absolute value of) an actual lateral acceleration acting on the vehicle is equal to or higher than a first threshold but is not greater than a second threshold greater than the first threshold. When the actual lateral acceleration (its absolute value) increases further and reaches the second threshold in spite of application of the inner rear wheel braking force, like the apparatus disclosed in the first mentioned publication, the motion control apparatus applies an outer wheel braking force.
By virtue of this configuration, in a process in which the roll angle increases, first only the inner rear wheel braking force is applied to the relevant wheel. As a result, a yawing moment in the same direction as the turning direction of the vehicle is generated in the vehicle, whereby the desired turning locus tracing performance of the vehicle can be maintained satisfactorily. In addition, on a portion of the vehicle body located above the rear wheel located on the inner side of a turning locus, a force for suppressing an increase in the height of that portion (hereinafter referred to as “vehicle height reducing force” acts, whereby an increase in the roll angle is suppressed. That is, it is possible to prevent excessive rolling of the vehicle, while satisfactorily maintaining the desired turning locus tracing performance.
Here, a mechanism in which the vehicle height reducing force is generated will be described. Such vehicle height reducing force is generated because a suspension which links a wheel and the vehicle body typically has a structure in which an instantaneous center located on the vehicle body side and which becomes effective when the wheel moves in relation to the vehicle body is located above and frontward in relation to the center of the wheel. That is, when a braking force is applied to a certain wheel, a braking force acts on the instantaneous center located on the vehicle body side (accordingly to the vehicle body itself), and a moment around the instantaneous center acts on that wheel. Because of the above-described positional relation between the center of the wheel and the instantaneous center, the moment acts in a direction of reducing the vertical distance between the center of the wheel and the instantaneous center. As a result, a force (that is, vehicle height reducing force) for suppressing an increase in the height of the instantaneous center located on the vehicle body side (accordingly, a portion of the vehicle body located above such a wheel) acts on that portion. In this manner, when a braking force is applied to a certain wheel, a vehicle height reducing force acts on a portion of the vehicle body located above that wheel.
In general, when a vehicle is in a deceleration state, the load imposed on the front wheels increases due to an inertial force acting on the vehicle. Accordingly, when a vehicle is in a deceleration state, a yawing moment is effectively generated in the vehicle upon application of a braking force to a front wheel, as compared to the case where a braking force is applied to a rear wheel. In addition, when a braking force is applied to the driven wheel located on the radially inner side of a turning locus, due to the action of a differential in a driven state, a proportion of the drive force distributed to the driven wheel located on the radially outer side of the turning locus increases, so that a yawing moment of the same direction as the turning direction can be generated effectively. In view of the above, the desired turning locus tracing performance is expected to be maintained more satisfactorily, if, instead of the inner rear wheel braking force, a force (hereinafter referred to as “inner front wheel braking force”) is first applied only to a front wheel located on the radially inner side of the turning locus. That is, the apparatus disclosed in the second mentioned publication has room for improvement in terms of properly maintaining the desired turning locus tracing performance.